This invention relates generally to the assembly of an optical element that has a response which varies with angle of incidence of light upon it, such as a dichroic filter element; the invention more particularly relates to a method of constructing and tuning a WDM filter during manufacture thereof and to a WDM itself.
Precisely aligning an optical fibre with an optical component such as a multilayer thin film optical interference filter, for example, a dichroic filter, which has a wavelength response that is dependent upon the angle of incident light, is generally a time consuming activity requiring a high degree of precision from an operator. One method of achieving a required alignment between an optical fibre end and a filter element, which has been used as of late, is to provide a plurality of different optical fibre ferrules or sleeves sometimes referred to as a fibre tubes, each having two spaced bores therein tightly accommodating two optical fibres. These sleeves are used for testing the response of an optical filter; and, each sleeve has a different spacing xe2x80x9cbxe2x80x9d between the pair of optical fibre ends contained therein. Hence, several sleeves having a pair of fibres with ends varying in lateral distance by an incremental amount are used for the testing procedure. Consequently, the smallest difference between different sleeves provides the smallest step or increment in which tunability is afforded. In order to test the dichroic optical filter element prior to its assembly into a coupler, different tubes are selected and used for testing. Broadband light is launched from one of the optical fibres in one of the tubes toward the angle sensitive optical filter; a detector at the output end detects the wavelength of light transmitted through the filter element, thereby identifying the tube from which the light originated. Different tubes are used to vary the input angle; as angle of incidence is varied, the wavelength of detected light varies, and hence so does the output response of the filter. When a wavelength of light measured at the output corresponds to a desired wavelength, the dimensions of the tube are noted and a tube of corresponding dimensions is used, or ordered from a supplier so that the filter may be assembled. Unfortunately, this method requires stocking a variety of sizes of optical fibre tubes, or alternatively, waiting for delivery of a supply of tubes having openings of desired separations. Notwithstanding, when a tube of a desired dimension is received its dimensions will sometimes vary from the expected dimension, due to manufacturing tolerances of the tubes, unduly affecting the response of the filter. Tuning resolution is also limited by the fact that tube bore separations vary discretely. In contrast, and advantageously this invention provides a means for providing continuous tuning.
It is desired to obviate the costly and time consuming process of manufacturing optical filters, by providing a means of tuning the filter during the testing process and fixing the components in place once a desired response is achieved.
It is an object of this invention, to provide such a method and filter which obviate the requirement of using such a large number of different sleeves in the manufacturing process.
It is a further object of the invention to provide an optical fibre sleeve that is used in testing of the filter, wherein the same sleeve is used in the manufacture of the filter, thereby obviating the requirement of providing a xe2x80x9cfacsimilexe2x80x9d sleeve to the test sleeve.
It is a further object of the invention to provide a tunable filter assembly that can be tuned during testing and affixed in place after the assembly elements are relatively tuned to achieve a desired response.
It is yet a further object of the invention to provide a coupler that can be continuously tuned, obviating the limitation of prior art methods of discrete tuning increments.
In accordance with the invention, there is provided, an optical filter comprising:
a first optical waveguide;
an optical filter element disposed to receive light from the first optical waveguide;
a first lens and a second lens disposed between the first optical waveguide and the optical filter, such that a beam of light exiting the first optical waveguide destined for the optical filter traverses the first lens and the second lens and is substantially collimated at the optical filter; and,
a second optical waveguide for receiving at least a portion of the beam of light after it impinges upon the optical filter element.
In accordance with another aspect of the invention a method of manufacturing an optic coupler is provided having two optical fibres at an end disposed within an optical fibre tube housing ends of the two optical fibres in a fixed relative relationship having a fixed lateral distance between the ends comprising the steps of:
a) providing an optical filter element and disposing the optical fibre ends to receive or provide light to the optical filter element;
b) providing a compound lens including at least two spaced lenses and disposing the compound lens between the optical fibre ends and the optical filter element; and,
c) varying a gap between the two spaced lenses to provide a desired wavelength response for light propagating from one of the ends of the two optical fibres through the filter.